1. Field of the Invention
The present invention relates to a method of determining a threshold arrangement for generating a gradation image and an apparatus for generating gradation image data, which are preferably applicable to a printing apparatus such as a color scanner, an image setter, a CTP (Computer To Plate) outputting machine, a CTC (Computer To Cylinder) printing press, a DDCP apparatus, etc.
2. Description of the Related Art
Halftone dot image outputting apparatus such as image setters or the like produce a halftone dot image of binary values (e.g., black and non-black areas produced by turning on and off a laser beam) on a printing sheet or a film. It has been pointed out that an image outputted by such a halftone dot image outputting apparatus suffers a moiré pattern produced by an interference between the output resolution of the apparatus and a screen ruling (see Japanese laid-open patent publication No. 8-317212).
The output resolution refers to the resolution of the image outputting apparatus, and is defined by dpi (dots per inch), pixels/inch which has the same meaning as dpi, or pixels/mm or the like. The screen ruling is defined by lines/inch, which can be converted into lines/mm, representing the number of rows of halftone dots (also referred to as halftone dot cells) per unit length (inch), and is also referred to as lpi (lines per inch), a ruling, a screen frequency, or a halftone dot frequency.
The moiré pattern produced by an interference between the output resolution and the screen ruling is a periodic pattern of halftone dots, i.e., a periodic interference pattern caused between a halftone dot pitch and a scanning line pitch. The moiré pattern acts as a low-frequency noise component that degrades the quality of the image.
The inventor of the present invention has proposed techniques to reduce the low-frequency noise component as disclosed in Japanese laid-open patent publication No. 8-317212 (hereinafter referred to as “first technique”), Japanese laid-open patent publication No. 9-200518 (hereinafter referred to as “second technique”), and Japanese laid-open patent publication No. 11-112814 (hereinafter referred to as “third technique”).
According to the first technique, the positions of thresholds in a threshold arrangement (also referred to as “threshold template” or “threshold matrix”) that is used to generate binary halftone dot image data are modified to equalize, as much as possible, the numbers of pixels to be blackened or unblackened in the threshold arrangement for thereby reducing the low-frequency noise component.
According to the second technique, when thresholds are arranged in the threshold arrangement according to the first technique, random numbers are added to further reduce the low-frequency noise component.
According to the third technique, the central value of a given threshold correction range in existing thresholds to be corrected in a threshold arrangement and the thresholds to be corrected are compared with each other for magnitude to convert the thresholds into halftone dot image data. The halftone dot image data are then converted into data in a frequency space, from which data containing a low-frequency noise component whose frequency is lower than the fundamental frequency component of halftone dots are extracted and converted into image data in a real space. The image data in the real space and the thresholds to be corrected are observed in the given threshold correction range, and a pair of thresholds to be replaced, i.e., a pair of thresholds at positions where pixels having maximum and minimum values of the image data in the real space, are selected and replaced with each other to obtain a corrected threshold arrangement.
The corrected threshold arrangement produced by the third technique is a threshold arrangement that is less likely to produce a low-frequency noise component.
While the first and second techniques can achieve respective certain reduced levels of the low-frequency component, they have been desired to accomplish a further reduced level of the low-frequency component if images of higher quality are handled.
The third technique is capable of considerably reducing the low-frequency noise component. However, since the third technique is a technique to correct already generated thresholds, the degree of freedom of correcting those thresholds is limited, resulting in a possibility that no sufficient noise reduction capability may be achieved.